Chromium plating



United States Patent '9 M CHROMHIM PLATING Thomas Blaine, Parma Heights, Ohio, assignor to The Harshaw Chemical Company, Cleveland, Ohio, a corporation of Ohio N Drawing. Application March 14, 1955, Serial No. 494,263

Claims. (Cl. 204-51) This invention relates to electrodeposition of chromium and specifically to electrodeposition of bright chromium on a surface of antimony, tin, or silver and semi-bright chromium on lead.

The art of chromium plating has developed commercially largely as a coating for nickel electrodeposits. The standard chromium bath for the production of bright chromium deposits on bright nickel surfaces contains, in addition to water, substantially chromic acid and a minor proportion of sulfate ion.

The standard chromium plating solution as used in nickel plating is capable of producing on antimony, tin or silver surfaces chromium deposits of limited adherence, but even on highly buffed antimony surfaces chromium deposits from the standard bath and of a thickness to protect the antimony adequately are not bright as taken from the solution but tend to be cloudy or gray and of course the hardness of chromium renders it diificult if not entirely impractical to buff the chromium deposit.

It has now been discovered in accordance with the present invention that by the use of a suitable chromium plating bath, differing from the above mentioned standard bath, a bright and more adherent chromium coating can be obtained on a bright surface of antimony, tin, or silver which will be fully bright as taken from the chrome plating solution without any bufiing or polishing of any kind and a semi-bright deposit (superior to that which is produced by the standard bath) can be made on polished lead, likewise without buffing or polishing the chromium deposit. The solution referred to is primarily an aqueous chromic acid solution containing a suitable concentration of acetic acid or a salt thereof supplying acetate ion, and a minor proportion of sulfate ion. The present preferred solution according to the invention contains chromic acid, sodium acetate, sulfate ion and water, the chromic acid being the principal constituent, suitably present to the extent of from 100 to 700 grams per liter, most desirably about 250 to 450 grams per liter; the acetate ion being present in concentration from about 15% to about 35% of the chromic acid by weight, most desirably from about 50 to about 100 grams per liter; sulfate ion being present approximately to the extent of from 0.3% to 1.0% of the chromic acid by weight, most desirably from about 1.0 to 3.0 grams per liter; the temperature of operation being in the order of 75 to 130 F., most desirably about 80 to 100 F.

While bright deposits can be obtained without the sulfate ion, it is found that thicker deposits of the same quality can be made without losing brightness if the sulfate ion is present, and, also, the bath operates in a higher current density range to produce brightness. Good deposits are obtained in a current density range from 100 to 400 amperes per square foot from the 2,7 45,801 Patented -May 15,- 1956 chromic acid-acetate bath containing a suitable proportion of sulfate ion. 7

Preferred chrome plating solutions according to 'th present invention are as follows:

F. e. g, 100 1 Cathode current density 200-300 (e. g., 200).

The following specific examples of aqueous chromium electroplating solutions will serve to illustrate the invention:

Example I C'I'Oa 250 grams per liter. Acetic acid a -a 50 grams .per liter. Sulfuric acid; -a 1.0 gram per liter. Cathode current density 200 A. S. F. Temperature Room to 100 F.

From this solution excellent bright deposits are produced on bright antimony, bright'tinand bright "silver and semibright deposits on polished lead to 'a thickness 'of0.02 niil.

Example II CrOa grams per liter. Acetic acid 200 grams per liter. Sulfuric acid 3 gramsp'er liter. Cathode current density 100 A. S. F. Temperature Room 't'o'l10' F.

Results are substantially as in Example II with the exception that current efliciency is improved by the use of sodium acetate instead of acetic acid.

Deposits taken from the foregoing solutions, the same being electrolyzed between a lead'anode and a bright antimony cathode are adherent on both compression and expansion bending and bright up to a thickness of 0.00002 inch. These baths have given continued good results after a considerable period of laboratory testing and plant scale testing. Deposits from standard chromium plating solutions do not resist compression bending as well and are not as bright at these thicknesses. The chromic acid, sodium acetate, sulfate ion solution is preferred although the sodium acetate can be replaced by acetic acid, other alkali metal acetates, ammonium acetate or other compounds compatible with the solution and supplying acetate ions.

Higher current efiiciency can be obtained by using acetate salts as against acetic acid. Better results are obtained by adding alkali to a bath containing acetic acid. (The preferred upper limit of alkali is an amount equivalent to 88 g./l. NazCOa over and above that needed to neutralize the acetic acid. K, Na, Mg, Ca, A1, Ni, and NH; salts have been tried and all are about equivalent when used in equivalent concentrations (mols per liter)).

It is an important feature of this invention that the deposits be on the specified surfaces, since the special solutions above described are not needed for electroplating on bright nickel surfaces and are not elfective on metallic surfaces in general. They do, however, give excellent results on bright'antimony, silver and tin surfaces vention.

This application is a continuation-in-part of my copending application Serial No. 436,233, filed June 11, 1954, which was a continuation-in-part of my application Serial No. 319,608, filed November 8, 1952, both :now abandoned.

, Having thus described the invention, what is claimed .1. A process capable of producing adherent chromium :deposits of improved lustre on bright surfaces of the class consisting of antimony, tin, silver and lead comprising electrolyzing between an anode surface and a bright cathode surface of the class consisting of bright antimony, bright tin, bright silver and bright lead surfaces, an aqueous solution essentially consisting of water, chromic acid, acetate ion and sulfate ion, acetate ion being present in concentration from about to about of the chromic acid by weight, sulfate ion being present to the extent of from about 0.3 to about 1.0 per 7 cent of the chromic acid by weight and chromic acid being present in concentration from about 100 to about 700 grams per liter.

'2. A process capable of producing bright adherent chromium deposits on bright surfaces of the class con sisting ofantimony, tin, silver and lead comprising electrolyzing, at a cathode current density from about 100 1 to about 400 amperes per square foot between an anode surface and a bright cathode surface of the class consisting of bright antimony, bright silver, bright tin and bright lead, an aqueous solution essentially consisting of water, chromic'acid, acetate ion and sulfate ion, acetate ion being present in concentration from about 15 to about 35% of the chromic acid by weight, sulfate ion being present from about 0.3 to about 1.0% of the chromic acid by weight, and chromic acid being present in concentration from about 100 to about 700 grams per liter.

3. A process capable of producing bright adherent chromium deposits on bright antimony surfaces comprising electrolyzing, at a cathode current density from about 200 to about 300 amperes per square foot between an anode surface and a bright antimony cathode surface, an aqueous solution essentially consisting of water, chromic acid, acetate ion and sulfate ion, acetate ion being present in concentration from about 15 to about 35% of the chromic acid by weight, sulfate ion being present from about 0.3 to about 1.0% of the chromic acid by weight, and chromic acid being present in concentration from about 250 to about 350 grams per liter.

4. A process capable of producing bright adherent chromium deposits on bright antimony surfaces comprising electrolyzing, at a cathode current density from about to about 400 amperes per square foot between an anode surface and a bright antimony cathode surface, an aqueous solution essentially consisting of water, chromic acid, acetate ion and sulfate ion, acetate ion being present in concentration from about 15 to 35% of the chromic acid by weight, sulfate ion being present from about 0.3 to about 1.0% of the chromic acid by weight, and chromic acid being present in concentration from about 100 to about 700 grams per liter, said solution being maintained at a temperature from about 75 F. to about 130 F.

5. A process capable of producing bright adherent chromium deposits ontbright antimony surfaces comprising electrolyzing, at a cathode current density from about 200 to about 300 amperes per square foot between an anode surface and a bright antimony cathode surface, an aqueous solution essentially consisting of water, chromic acid, sodium acetate and sulfuricacid, sodium acetate being present in concentration molecularly equivalent to acetic acid from about 15 to 35% of the chromic acid by weight, sulfate being present from about 0.3 to about 1.0% of the chromic acid by Weight, and chromic acid being present in concentration from about 250 to about 350 grams per liter, said solution being maintained at a temperature from about 75 F. to about F.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A PROCESS CAPABLE OF PRODUCING ADHERENT CHROMIUM DEPOSITS OF IMPROVED LUSTRE ON BRIGHT SURFACES OF THE CLASS CONSISTING OF ANTIMONY, TIN, SILVER AND LEAD COMPRISING ELECTROLYZING BETWEEN AN ANODE SURFACE AND A BRIGHT CATHODE SURFACE OF THE CLASS CONSISTING OF BRIGHT ANTIMONY, BRIGHT TIN, BRIGHT SILVER AND BRIGHT LEAD SURFACES, AN AQUEOUS SOLUTION ESSENTIALLY CONSISTING OF WATER, CHROMIC ACID, ACETATE ION AND SULFATE ION, ACETATE ION BEING PRESENT IN CONCENTRATION FROM ABOUT 15 TO ABOUT 35% OF THE CHROMIC ACID BY WEIGHT, SULFATE ION BEING PRESENT TO THE EXTENT OF FROM ABOUT 0.3 TO ABOUT 1.0 PER CENT OF THE CHROMIC ACID BY WEIGHT AND CHROMIC ACID BEING PRESENT IN CONCENTRATION FROM ABOUT 100 TO ABOUT 700 GRAMS PER LITER. 